Body implantable stimulators are known to the prior art, the most common being the well-known cardiac pacemaker. Within such stimulators, advances in power source and electronic technology have significantly reduced in the volume required by the signal generating components while providing increases in power source life and an enhanced adaptability of the operating characteristics.
While the reduction in volume of the signal generating components affords a tremendous opportunity to reduce the size of the stimulator itself, that opportunity is restricted by the prior art interconnection between the signal generator and the lead. For example, in the typical prior art bipolar body stimulator it has been a common practice to employ a bifurcated lead with the lead furculae being inserted in side-by-side relation within a connector assembly. However, the available volume reduction of the signal generator allows a reduction in thickness to a dimension which is unable to accommodate the prior art lead furculae in side-by-side relation. Therefore, to take advantage of the potential for reduced signal generator thickness it has been suggested that the lead connector accept the furculae at different distances from the signal generator in a "over/under" relationship. That is, it has been suggested that the connector accept one furcula at a position relatively adjacent the signal generator and the second furcula at a position more distant from the signal generator such that the thickness requirement of the lead connector can be compatible with the potential reduced thicknesss of the signal generator. Alternatively, it has been suggested that the diameter of at least the lead furculae be reduced such that the prior art side-by-side arrangement may be continued. However, inasmuch as the stress within a lead varies geometrically in accordance with its dimensions (radius, for example) a significant reduction in size necessarily results in an even more significant, and perhaps prohibitive, increase in stress concentration in the lead. In addition, alteration in the size of lead from prior art standards will result in the necessity of replacement of a lead on replacement of a signal generator assembly or in the use of adapters, neither concept having a high degree of acceptability. Accordingly, the use of a "over/under" arrangement appears to have the greatest potential for acceptance.
Typical prior art bifurcated leads have furculae of equal length. In addition, prior art lead systems are made to standardized lengths often resulting in the use of a lead of greater length than is necessary in a particular implant. The excess length is typically taken up by wrapping the excess lead and furculae around the signal generator assembly prior to placement of the signal generator assembly in the "pocket". In view of the equal furcula lengths of existing lead assemblies, this "wrapping" has led to difficulties in the conversion from a "side-by-side" to a "over/under" connector arrangement. That is, a mere reconfiguration of a connector from a "side-by-side" to a "over/under" relationship results in an excess furcula length in the furcula closest to the signal generator, the excess length interfering with a proper wrapping of the upper furcula around the signal generator assembly. In addition, the excess length of the furcula closest to the signal generator produces pockets which encourage fibrosis. Thus, while a "over/under" arrangement allows the use of existing leads systems with a signal generator of reduced thickness, wrapping and fibrosis problems result from that arrangement.